The disclosure herein relates to pressure measurement apparatus, such as, for example, used in extra-corporeal blood treatment. More particularly, the disclosure relates to diaphragm repositioning in such pressure measurement apparatus (e.g., repositioning the diaphragm of a pressure pod such that valid pressure readings are obtainable).
Extracorporeal blood sets, for example, are used in a variety of medical procedures to treat patients, such as, the infusion of drugs, dialysis, continuous renal replacement therapy (CRRT), extracorporeal membrane oxygenation (ECMO), etc. Reducing cost while maintaining safety and accuracy are of concern in today's healthcare environment. Reducing the number of tasks a user must perform and/or monitoring tasks performed such that the tasks are completed correctly, reduces the cost of operation and increases the quality of health care.
In many extracorporeal blood sets (e.g., disposable blood sets) provided, for example, for use in therapy systems, pressure pods are used to separate the liquid/blood filled disposable extracorporeal circuit from an electronic pressure sensor of the system by preventing liquid ingress and contamination while enabling the transfer and measurement of pressure. Such pressure pods may include a pressure transducer side separated from a fluid flow side by a diaphragm. In one or more configurations, for example, the pressure transducer side of the pressure pod is filled with air in a sealed space providing isolation (e.g., electrical isolation) thereof from the fluid flow side and a medium for the transfer of pressure from the fluid flow side to the pressure transducer side of the pressure pod, e.g., the compression of air. For example, the diaphragm which separates the pressure transducer side from the fluid flow side of the pressure pod may be flexible and oversized to ensure none of the force exerted by the pressure on the diaphragm in the extracorporeal blood circuit is lost to the tension or compression of the diaphragm. Further, for example, the pressure pod (e.g., via the pressure transducer side of the pressure pod) may be operatively connected by tubing (e.g., air filled) to a pressure transducer for sensing pressure at a distance away from the pressure pod (e.g., a pressure transducer located in a system housing upon which the extracorporeal blood set is mounted or a system to which the extracorporeal blood set is connected).
For example, a disposable extracorporeal blood set connectable to a treatment system (e.g., mounted on a system housing and connected to one or more pressure transducers therein) may contain multiple pressure pods. Each pressure pod may contain a diaphragm that separates a liquid (e.g., blood in the fluid side of the pressure pod) from an air cavity (e.g., on the transducer side of the pressure pod) and which is configured to fit into a pressure sensor or pod receptacle of the system (e.g., a connection apparatus for mounting the pressure pod on a dialysis unit). The pressure pods and pressure transducers (e.g., transducers inside the control unit or system housing, such as a dialysis unit) enable noninvasive pressure monitoring of the liquid (e.g., blood), since the liquid never comes into contact with the actual pressure transducer. A pressure pod receptacle may be used to provide connection between the pressure pods and the pressure transducers within the system housing. For example, the pressure pods may be manually attached to the pressure pod receptacle by a user each time a new disposable blood set is installed.
Since air is compressible and follows the ideal gas law under low pressures which exist in the extracorporeal blood circuit, the diaphragm position is a function, for example, of the atmospheric pressure, the volume of air in the closed space encompassing the air volume of the pressure transducer, any tubing volume between the pressure transducer and pressure pod, the elasticity of the tubing, and the volume of air in the pressure pod. As the circuit pressure increases and decreases in the liquid path during therapy such as dialysis, the position of the diaphragm will change accordingly. For example, under negative pressure, the flexible membrane, e.g., the diaphragm, will deflect towards the blood portion, e.g., liquid flow side, of the pressure pod and, for example, during positive pressure, the flexible membrane will flex toward the air side or pressure transducer side of the pressure pod.
However, if there is too little or too much air volume in the pressure transducer side, i.e., the air side, of the pressure pod due to, for example, a leak, a change in temperature, a change in blood pressure, or a change in atmospheric pressure, the potential exists for the flexible diaphragm to touch the pod casing on the liquid flow side of the pressure pod (e.g., top out) or come under tension (e.g., due to the slack in the flexible diaphragm being used) and bottom out (e.g., touch the pod casing on the transducer side of the pressure pod) resulting in an incorrect pressure reading because the true circuit pressure is no longer being transmitted. Conventionally, medical device systems have overcome such limitations by, for example, alerting the user to changes in pressure or at set periods of time to request the user to check the diaphragm position and/or to enable a repositioning of the diaphragm by the user. Such a check and/or reposition procedure takes user time and also may momentarily disable pressure measurement during the procedure (e.g., during therapy being provided to a patient).
For example, during a software initiated periodic check and/or reposition procedure carried out by a user (e.g., see the Open Loop Diaphragm Repositioning Sequence described herein), the diaphragm position may be adjusted back to a centered measuring position by infusing air to or withdrawing air from the enclosed space on the transducer side of the pressure pod. The trapped volume of air within the pressure pod is a known volume and by flexing the diaphragm under positive and negative pressure, the extension limits of the flexible diaphragm may be found by examining the rate of the change in pressure. For example, when the diaphragm deflection is halted due to tension or due to the diaphragm coming into contact with the sides of the pod (e.g., topped out or bottomed out on the pod casing), the rate of change of pressure will dramatically increase because the compliance of the chamber decreases, where compliance is measured in terms of pressure change per change in volume of air. Once both the positive and negative extension limits are determined, the centered measuring position may be found by infusing a known volume of air into the closed system (e.g., by activating a valve and connecting a positive displacement air pump to the enclosed space on the transducer side of the pressure pod).
In other words, for example, a disposable extracorporeal blood set connectable to a therapy system (e.g., mounted on a system housing and connected to one or more pressure transducers therein), may contain multiple circular pressure pods. Each pressure pod may contain a diaphragm that separates a liquid (e.g. blood in the liquid side of the pressure pod) from an air cavity (e.g., on the transducer side of the pressure pod) and which is configured to fit into a pressure sensor housing on a control unit (e.g., a mating receptacle for mounting the pressure pod on a dialysis unit). The pressure pods and pressure transducers (e.g., inside the control unit, such as a dialysis unit) enable noninvasive pressure monitoring of the liquid (e.g., blood), since the liquid never comes into contact with the actual pressure transducer. However, for the sensor to yield valid pressure readings, the pressure pod diaphragm must stay in a center range of the pressure pod. This may be accomplished by using an air pump (e.g., of a pump system) to add air to or remove air from the pressure pod air cavity (e.g., on the transducer side of the pressure pod) such that the air pressure on the air side of the diaphragm (e.g., the transducer side of the pressure pod) is equal to the liquid pressure on the other side of the diaphragm (e.g., the liquid flow side of the pressure pod). This may be referred to as having the pod diaphragm “in the measuring position” (e.g., a target measuring position).
Two conventional methods that may be used to move the diaphragm to the centered position include an Open Loop Diaphragm Repositioning Sequence and a Research of Plateau Repositioning Sequence. For example, an Open Loop Diaphragm Repositioning Sequence may be performed as follows. Periodically, an air pump may be operated to either add or remove air such that the pressure transducer readings from a given pressure pod is increased or decreased by 100 mmHg. If the initial pressure difference between the air cavity pressure and liquid pressure is small, then the diaphragm should be pushed against one of the pressure pod walls (e.g., on the transducer side or the liquid flow side of the pressure pod). This is referred to as the diaphragm either being bottomed out (e.g., minimum air cavity volume) or topped out (e.g., maximum air cavity volume). Then the pump may be operated to add or remove air volume equal to ½ the total volume of the pod. If the diaphragm was either bottomed out or topped out, this should center the diaphragm in the pod. However, if the diaphragm was not actually bottomed out or topped out, then it will not be centered after the open loop diaphragm repositioning sequence. Numerous conditional checks (e.g., such as calculating the derivative of the pressure readings while the pump is adding or removing air) are done to determine success or failure of the open loop repositioning sequence. If these checks indicate a failure, then a Research of Plateau Test Sequence may be executed. If the checks indicate success, then the repositioning sequence for the given pod may be terminated.
The Research of Plateau Repositioning Sequence may be performed as follows. This sequence may be executed if automated checks indicate that the open loop diaphragm repositioning sequence failed. In this sequence, the air pump is again used to add/remove air to/from the pod air cavity (e.g., on the transducer side of the pressure pod). In this case, however, the derivative of the pressure transducer reading is calculated while the pump is adding/removing air at a constant rate. If the diaphragm is in the measuring range, then the pressure derivative magnitude will be small. When the diaphragm reaches either a bottomed out or topped out condition, however, the pressure derivative magnitude increases beyond a threshold, indicating that the diaphragm has reached one wall of the pressure pod. At that point, the pump direction may be reversed and continue to operate until the pressure derivative again exceeds a threshold indicating that the diaphragm has contacted the opposite wall of the pressure pod. The air pump may again be reversed to add or remove an air volume equal to half of the volume required to move the diaphragm from the initial pod wall contact to the opposite pod wall contact. The diaphragm should then be centered in the pod and pressure readings from the pressure sensor (e.g., pressure measurements) should be valid.
Further, for example, the position of the diaphragm may also be manually repositioned by a user. For example, based upon the user visually examining the position of the diaphragm, the user may infuse air or remove air from the system to center the diaphragm (e.g., the user may control the pump to infuse or remove air).
However, as mentioned herein, such processes (e.g., processes that, for example, request the user to check the diaphragm position at set periods of time) undesirably take user time to perform. Further, the dependence on pressure derivative calculations make the process sensitive to pressure sensor random noise and periodic pressure peaks resulting from, for example, peristaltic pumps used to pump liquids in a system where pressure measurements may be taken. Still further, heavy digital filtering may be required to reduce the resulting pressure derivative peaks to a level that does not cause false triggering of the various Open Loop Diaphragm Repositioning Sequence and Research of Plateau Repositioning Sequence tests. Such filtering may result in up to two seconds of latency in obtaining the filtered pressure and pressure derivative used in the repositioning sequence. Further, the repositioning sequence may take up to three minutes per pressure pod and may be repeated if the first attempt fails. During this time, therapy pressure monitoring using the pressure pod under test may not be available even though the patient therapy or treatment is continuing. Yet further, with two different sequences (e.g., Open Loop and Research of Plateau processes) being used, with decision criteria being used to decide whether to start the sequence by either adding or removing air, and with decision criteria being used to determine success or failure of either sequence, the algorithms for implementing the sequences may be complex.